Monodisperse polymer particles (i.e. particles with a coefficient of variation of less than 10%, preferably less than 5% and more preferably less than 3%) have been commercially available for several years and find applications in many technical fields, e.g. in pharmaceuticals, in separation processes, as toners, as filters, as spacers, etc.
Polymer beads may be produced by diffusing a monomer and a polymerization initiator (or catalyst) into polymer seeds in an aqueous dispersion. The seeds swell and following initiation of polymerization, e-g. by heating to activate the initiator, larger polymer particles are produced. The maximum volume increase due to swelling and polymerization is about ×5 or less. The late Professor John Ugelstad found that the capacity of the seeds to swell could be increased to a volume increase of ×125 or even more if an organic compound with relatively low molecular weight and low water solubility is diffused into the seeds before the bulk of the monomer is used to swell the seeds. The effect is based on entropy and not particularly in the chemical nature of the organic compound. Conveniently the polymerization initiator may be used for this purpose. Organic solvents, e.g. acetone or a relatively small portion of the monomer, may be used to enhance diffusion of the organic compound into the seeds. This “Ugelstad polymerization process”, which is described for example in EP-B-3905 (Sintef) and U.S. Pat. No. 4,530,956 (Ugelstad), may be used to produce monodisperse particles, if necessary carrying out several swelling and polymerization stages to reach the desired particle size.
WO 92/16581 (Cornell Research Foundation) also describes the preparation of monodisperse particles, particularly macroporous polymer beads. The process described uses a three phase emulsion containing soluble polymer particles, a monomer phase, and water. The three phase emulsion also includes an emulsifier and a suspension stabilizer. The polymer particles undergo swelling absorbing the monomer which is then polymerized. In this process the soluble polymer seed particles act as both shape/size regulators and as a porogen. The initial (i.e. before swelling) particles have a diameter of from about 0.5 to 10 μm, 2 to 5 μm being most preferred, and are produced by conventional techniques, such as emulsion or dispersion polymerization.
In a simplified version of the Ugelstad process the enhanced capacity for swelling may be achieved simply by the use of oligomeric seed particles, e.g. where the oligomer weight average molecular weight corresponds to up to 50 monomer units or up to 5000 Dalton.
The processes described in EP-B-3905 and U.S. Pat. No. 4,530,956 (the disclosures of which are hereby incorporated by reference) and the simplified Ugelstad process are relatively complex and inefficient. The processes described in WO 92/16581 do not especially improve upon those disclosed in EP-3-3905 and U.S. Pat. No. 4,530,956. The essence of WO 92/15681 would appear to be the production of macroporous polymer beads of substantially uniform size, the macroporosity being achieved through extraction of the (initially) soluble polymer from the resultant insoluble expanded beads. It is well known in the art that addition of steric stabilizers to dispersion polymerizations of polymer seeds can be useful in controlling size of beads; this feature of WO 92/16581, therefore, appears to represent nothing more than the arbitrary introduction of an obvious and well-known advantageous process feature into the process of the invention.
There is a need for improvements to all these processes, in particular improvements which make it easier to produce monodisperse polymer particles with different chemical or physical characteristics.